The mobile communication system has evolved into a high-speed, high-quality wireless packet data communication system to provide data and multimedia services beyond the early voice-oriented services. Recently, various mobile communication standards, such as High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE), and LTE-Advanced (LTE-A) defined in 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) defined in 3rd Generation Partnership Project-2 (3GPP2), and 802.16 defined in IEEE, have been developed to support the high-speed, high-quality wireless packet data communication services. Particularly, LTE is a communication standard developed to support high speed packet data transmission and to maximize the throughput of the radio communication system with various radio access technologies. LTE-A is the evolved version of LTE to improve the data transmission capability.
Typically, LTE base stations and terminals are based on 3GPP Release 8 or 9, while LTE-A base stations and terminals are based on 3GPP Release 10. The 3GPP standard organization is preparing for the next release for more improved performance beyond LTE-A.
The existing 3rd and 4th generation wireless packet data communication systems (such as HSDPA, HSUPA, HRPD, and LTE/LTE-A) adopt Adaptive Modulation and Coding (AMC) and Channel-Sensitive Scheduling techniques to improve the transmission efficiency. AMC allows the transmitter to adjust the data amount to be transmitted according to the channel condition. That is, the transmitter is capable of decreasing the data transmission amount for bad channel conditions so as to fix the received signal error probability at a certain level, or increasing the data transmission amount for good channel conditions so as to transmit large amounts of information efficiently while maintaining the received signal error probability at an intended level. The channel sensitive scheduling allows the transmitter to serve the user having a good channel condition selectively among a plurality of users so as to increase the system capacity as compared to allocating a channel fixedly to serve a single user. This increase in system capacity is referred to as multi-user diversity gain. Both the AMC and channel sensitive scheduling are the method of adopting the best modulation and coding scheme at the most efficient time based on the partial channel state information feedback from the receiver.
A User Equipment (UE) is capable of performing the channel state information feedback to an evolved Node B (eNB) in one of a periodic CSI report mode or aperiodic CSI report mode. In the periodic CSI, the UE reports the channel state information periodically. The channel state information may include at least one of Rank Indication (RI), Precoding Matrix Index (PMI), and Channel Quality Indicator (CQI). In the aperiodic CSI report, the UE reports the channel state information in response to the request from the eNB.
In case of using AMC along with Multiple Input Multiple Output (MIMO) transmission scheme, it may be necessary to take a number of spatial layers and ranks for transmitting signals into consideration. In this case, the transmitter determines the optimal data rate in consideration of the number of layers for use in MIMO transmission as well as coding rate and modulation scheme.
FIG. 1 is a graph illustrating time-frequency resources in a LTE/LTE-A system.
As shown in FIG. 1, the radio resource for transmission from the evolved Node B (eNB) to a User Equipment (UE) is divided into Resource Blocks (RBs) in the frequency domain and subframes in the time domain. In the LTE/LTE-A system, an RB consists of 12 consecutive carriers and has a bandwidth of 180 kHz in general. A subframe consists of 14 OFDM symbols and spans 1 msec. The LTE/LTE-A system allocates resources for scheduling in units of subframes in the time domain and in units of RBs in the frequency domain.
FIG. 2 is a time-frequency grid illustrating a single resource block of a downlink subframe as a smallest scheduling unit in the LTE/LTE-A system.
The radio resource depicted in FIG. 2 is of one subframe in the time domain and one RB in the frequency domain. The radio resource consists of 12 subcarriers in the frequency domain and 14 OFDM symbols in the time domain, i.e., 168 unique frequency-time positions. In LTE/LTE-A, each frequency-time position is referred to as Resource Element (RE). One subframe consists of two slots, and each slot consists of 7 OFDM symbols.
The radio resource structured as shown in FIG. 2 can be configured to transmit different types of signals as follows:                CRS (Cell-specific Reference Signal): A reference signal broadcast within a cell at every subframe for use, at all the UEs within the cell, in channel estimation between the eNB and UE, monitoring radio link for validity, and fine tuning of time or frequency at baseband;        DMRS (Demodulation Reference Signal): A reference signal transmitted to a specific UE;        PDSCH (Physical Downlink Shared Channel): A data channel transmitted in downlink, which the eNB uses to transmit data to the UE, and mapped to REs not used for reference signal transmission in the data region of FIG. 2;        CSI-RS (Channel Status Information Reference Signal): A reference signal transmitted to the UEs within a cell and used for channel state measurement. Multiple CSI-RSs can be transmitted within a cell;        Other control channels (PHICH, PCFICH, PDCCH): Channels for providing control channels necessary for the UE to receive PDCCH, and transmitting ACK/NACK of HARQ operation for uplink data transmission.        
In addition to the above signals, zero power CSI-RS may be configured in order for the UEs within the corresponding cells to receive the CSI-RSs transmitted by different eNBs in the LTE-A system. The zero power CSI-RS can be applied to the positions designated for CSI-RS, and the UE receives the traffic signal on the resource excluding the zero power CSI-RS positions in general. In the LTE-A system, the zero power CSI-RS is referred to as ‘muting.’ This is because the muting by nature is mapped to the CSI-RS positions without transmission power.
In FIG. 2, the CSI-RS can be transmitted at some of the positions marked by A, B, C, D, E, F, G, H, I, and J according to the number of number of antennas transmitting CSI-RS. Also, the zero power CSI-RS (muting) can be mapped to some of the positions A, B, C, D, E, F, G, H, I, and J. The CSI-RS can be mapped to 2, 4, or 8 REs according to the number of the antenna ports for transmission. For two antenna ports, half of a specific pattern is used for CSI-RS transmission; for four antenna ports, all of the specific pattern is used for CSI-RS transmission; and for eight antenna ports, two patterns are used for CSI-RS transmission. Muting is performed by pattern. That is, although the muting may be applied to plural patterns, if the muting positions mismatch CSI-RS positions, it cannot be applied to one pattern partially. However, if the CSI-RS positions match the zero power CSI-RS (muting) positions, the muting can be applied to a part of one pattern.
In the case of transmitting CSI-RS for two antenna ports, the CSI-RS is mapped to two REs contiguous in the time domain with the orthogonal codes for distinguishing between the antenna ports. In the case of transmitting CSI-RS for four antenna ports, two REs are added for another two antenna ports to transmit the CSI-RS in the same way. The CSI-RS transmission for eight antenna ports is performed in the same way.
In a cellular system, the reference signal is transmitted for downlink channel state measurement. In the case of the 3GPP LTE-A system, the UE measures the channel state based on the CSI-RS transmitted by the eNB. The channel state is measured in consideration of a few factors including downlink interference. The downlink interference includes the interference caused by the antennas of neighbor eNBs and thermal noise that are important in determining the downlink channel condition. For example, in the case that the eNB with one transmit antenna transmits the reference signal to the UE with one receive antenna, the UE determines energy per symbol (Es) that can be received in downlink and interference amount (Io) that may be received for the duration of receiving the corresponding symbol to calculate Es/Io from the received reference signal. The calculated Es/Io is reported to the eNB such that the eNB determines the downlink data rate for the UE.
In the LTE-A system, the UE feeds back the information on the downlink channel state for use in downlink scheduling of the eNB. That is, the UE measures the reference signal transmitted by the eNB in downlink and feeds back the information estimated from the reference signal to the eNB in the format defined in LTE/LTE-A standard. In LTE/LTE-A, the UE feedback information includes the following three indicators:                RI (Rank Indicator): The number of spatial layers that can be supported by the current channel experienced at the UE;        PMI (Precoding Matrix Indicator): A precoding matrix recommended by the current channel experienced at the UE;        CQI (Channel Quality Indicator): A maximum possible data rate that the UE can receive signals in the current channel state. CQI may be replaced with the SINR, maximum error correction code rate and modulation scheme, or per-frequency data efficiency that can be used in similar way to the maximum data rate.        
The RI, PMI, and CQI are associated with each other in meaning. For example, the precoding matrix supported in LTE/LTE-A is configured differently per rank. Accordingly, the PMI value ‘X’ is interpreted differently for the cases of RI set to 1 and RI set to 2. Also, when determining CQI, the UE assumes that the PMI and RI which it has reported are applied by the eNB. That is, if the UE reports RI_X, PMI_Y, and CQI_Z, this means that the UE is capable of receiving the signal at the data rate corresponding to CQI_Z when the rank RI_X and the precoding matrix PMI_Y are applied. In this way, the UE calculates CQI with which the optimal performance is achieved in real transmission under the assumption of the transmission mode to be selected by the eNB.